A duplex hydraulic actuator system of the prior art may have two pistons disposed on the same output shaft. Each piston is associated with its own pair of chambers on either side of the piston. Each pair of chambers is controlled by a spool valve which is a valve that simultaneously connects one of the pair of chambers to an input fluid flow path and the other of the pair of chambers to an output fluid flow path. In this way, a pressure imbalance between the two chambers of a pair will push the associated piston in a first direction.
Both pistons should be actuated in the same direction at the same time. That is, both spool valves should be actuated simultaneously such that each spool valve directs fluid flow within the chambers such that both pistons are biased in the same direction. If the spool valves are not actuated simultaneously then one piston may be being biased in a first direction while the second piston may either be biased in the second direction (due to the other spool valve being open in the opposite direction) or be blocked (due to the other spool valve being closed)—this situation is called “force fight”. Either situation may cause increased pressure in the piston chambers and its associated galleries within the actuator. This can damage these components or can require them to be made extra strong (and heavy) to account for the possibility of force fight.